Nitrogen is used both in its gaseous state and its liquid state for many purposes in various industries. Gaseous nitrogen is commonly used for ambient temperature preservation of fruits and vegetables, the fracturing and stimulation of oil and gas wells for enhanced recovery purposes, the blanketing, purging, and drying of vessels and pipelines, and so on. Liquid nitrogen is used for purposes such as for the flash freezing of various foods, including chicken, beef, and coffee, the shrink fitting of metal pans with close and exacting tolerances, cryogenic grinding of metals, freezing of used automotive tires for grinding and recycling, freezing of earth for enhanced excavation purposes, cooling of cement for quicker curing, and so on. Oxygen, in its gaseous state, is used for medical purposes, such as in hospitals, and is used for commercial purposes, such as aerating water at fish farms for enhanced fish growth, in the fabrication of steel, in welding operations, the primary treatment of sludge in sewage treatment plants, and so on.
Various methods exist for producing nitrogen gas and oxygen gas, which gases can subsequently be cooled to their respective liquid states. Such methods will be discussed in greater detail subsequently. Each of these methods involves the use of ambient air as a source for the nitrogen gas and the oxygen gas, and accordingly, that source is a gaseous air mixture comprising about 78% nitrogen, 21% oxygen, less than 1% argon, 300 to 500 parts per million carbon dioxide and other gaseous elements and compounds in trace amounts.
One method of producing the nitrogen gas and oxygen gas involves a cryogenic process wherein compressed air, at a pressure in a range from about 70 psig to about 140 psig, is used. The compressed air is cooled in a water-to-air heat exchanger, and is then passed through an ambient temperature adsorbent, and/or a varying cryogenic temperature heat exchanger, for purposes of purification by way of carbon dioxide and water vapour removal. The purified gaseous mixture is then passed through another cryogenic temperature heat exchanger, a sub-cooler, and then through expansion turbines and/or expansion valves. It is then received into a rectification column, or rectification columns, in both liquid and gaseous forms, which distillation columns contain a number distillation trays for the purpose of separating the nitrogen, oxygen, and argon, one from the other, both in gaseous and liquid forms, where appropriate. Nitrogen, oxygen, and argon are subsequently drawn off the rectification column at precise points in order to ensure maximum contained purity of the particular gas or liquid being drawn off. The oxygen gas produced in this manner is about 95% to about 99.5% pure and is marketed in this form for virtually any suitable application. The nitrogen gas or liquid produced in this manner is 99.999% pure and can be used for virtually any suitable application.
While nitrogen gas or liquid nitrogen having a purity of 99.999% is necessary for some applications, such a high purity it is not necessary for all applications. It is known that 90% to about 99% pure nitrogen gas is quite suitable for some applications. For instance, oil refineries use 95% to about 97% pure nitrogen gas for drying and purging purposes, such as drying pipelines until they are substantially dry, and then use a smaller amount of 99.999% pure nitrogen gas to finish the drying and purging process. The advantage in this procedure is that the largest quantity of nitrogen, namely the 95% to about 97% pure nitrogen, is less expensive to produce than the 99.999% pure nitrogen. The 95% to 97% pure nitrogen gas that is used by refineries in the above described mariner, is produced by removing oxygen from air with natural gas or oil combustion methods. However, the 95% to 97% nitrogen gas could also contain up to about 2% to 3% oxygen, respectively, as well as other impurities, specifically water vapour and carbon dioxide, as is further discussed below. Nitrogen gas having a purity of 95% to 97% may also be produced in other manners including by membrane separation, or by pressure swing adsorption (PSA). Those other methods of nitrogen gas production will now be discussed in greater detail.
Membrane separation is one process for nitrogen gas production. In this non-cryogenic ambient temperature process, air is fed through an elongate cylinder having a hollow central bore, and having an amount of membrane fibre filtering material around the hollow central bore. The membrane fibre filtering material is made from various monomers and polymers, and exhibits the characteristic of diffusing water vapour, oxygen, and nitrogen, at different rates. Accordingly, most of the water vapour, and a substantial amount of the oxygen, filter through the membrane, while very little of the nitrogen filters through the membrane. The nitrogen escapes the cylinder through an opening at the far end thereof. The nitrogen gas that escapes is 90% to 99% pure, with therefore about 10% to 1% oxygen and minute amounts--measured in parts per million (p.p.m.)--of carbon dioxide, and H.sub.2 O as water vapour.
Pressure swing adsorption technology was commercialized in the mid-1970's for the ambient temperature non-cryogenic production of nitrogen from a compressed air feed stream. The critical component of the pressure swing adsorption process is a carbon molecular sieve. This adsorbent exhibits a significantly higher time dependent loading factor for oxygen than nitrogen, even though the equilibrium loading for both gases at pressures up to 145 psig is almost the same. Carbon molecular sieves have a pore structure smaller than 10 Angstroms. The inner surface of the sieve will be accessible to gases with the smallest molecular size. The carbon molecular sieve material has many micropores in the size range of the oxygen molecule, thus allowing oxygen molecules to easily enter the sieve. The slightly larger nitrogen molecules take more time to penetrate the micropores and be absorbed.
In a typical pressure swing adsorption nitrogen plant, compressed air is cooled to ambient temperature and flows into an air buffer tank which acts as an air reservoir during molecular sieve bed tower repressurization and also as an additional water removal point in the system. From the air tank, the air flows through one of two, alternating on-line pressure swing adsorption towers where it is dried and separated. Gas separation takes place as the air passes through the carbon molecular sieve. Product gas exits the top of the tower and is removed From the unit into the user's nitrogen supply system. While the onstream tower is producing nitrogen, the other is at atmospheric pressure and is regenerated by heating and exhausting the desorbed gases to the atmosphere. The tower switching process entails closing the feed and delivery valves, pressure equalization between the two towers, repressurization of the bottom or the offstream tower with air from the air buffer tank and the top of the offstream tower with nitrogen product from the downstream receiver tank, and depressurization of the tower going offstream. Adherence to purity specification is maintained by an oxygen analyzer controlling dump/delivery valves in the product nitrogen line.
Productivity (volume of nitrogen/volume of carbon molecular sieve) of a pressure swing adsorption plant is dependent on nitrogen purity. A given size pressure swing adsorption unit can produce over twice as much nitrogen at 95% purity than at 99.5% with only a 25% to 35% increase in feed air requirement.
As noted above, it is often preferable, where permissible as may be determined according to other criteria, to use nitrogen gas with a purity of less than 99.999% where possible since it is less expensive than 99.999% pure nitrogen gas produced by rectification.
Nitrogen gas having a purity of between 90% and about 99%, and which therefore contains about 10% to about 1% oxygen, respectively, can be converted into its liquid state, for various uses. The specific purposes of the present invention deal with the production of liquid nitrogen from nitrogen gas having a purity of between 90% and about 99%.
It is well known to produce 99.999% pure liquid nitrogen by means of a rectification column, as discussed above, and to use the 99.999% pure liquid nitrogen for various applications.
It is believed that it would be acceptable to use 90% to 99% pure liquid nitrogen for many applications where 99.999% pure liquid nitrogen is now used; however, it is not known to do so for the following reason: The 95% to 97% pure nitrogen gas produced by natural gas or oil combustion methods generally has in it certain impurities, including particularly H.sub.2 O and carbon dioxide. These impurities are present in sufficient amounts to make it difficult and expensive to liquify the 95% to 97% pure nitrogen gas. Moreover, it is prohibitively expensive to remove these impurities prior to liquification. Accordingly, it is not known to use 95% to 97% pure nitrogen gas produced by natural gas or oil combustion methods to produce liquid nitrogen, especially because of the presence of H.sub.2 O and CO.sub.2 impurities.
It is also not known to use 90% to 99% pure nitrogen gas produced by the use of membrane fibre filtering materials, or PSA, as discussed above, to produce liquid nitrogen.
Therefore, it is not known to use 90% to 99% pure nitrogen gas, whether it is obtained by combustion, PSA, or from membrane separation techniques, in order to produce liquid nitrogen.
What the present invention provides is a means, both by way of methods and apparatus, for producing liquid nitrogen from 90% to 99% pure nitrogen gas that has been derived from ambient air using membrane separation and/or molecular sieve separation techniques.